Hollow casting and manufacturing method therefor, and method and apparatus for smoothing internal surface of hollow casting

ABSTRACT

There is provided a hollow casting in which a surface roughness Ra of a casting wall portion surface forming a hollow portion formed by a wall portion is 10 μm or less. Such a hollow casting can be obtained by a manufacturing method for the hollow casting in which after molten metal is poured into a mold to be molded by casting, processing of removing residual matters on the casting wall portion surface forming the hollow portion and processing of smoothing the casting wall portion surface forming the hollow portion are performed at the same time under predetermined conditions. According to the present invention, a casting which has the hollow portion formed by the wall portion and whose wall portion surface forming the hollow portion is very smooth and which has a low passage resistance can be manufactured at a lower cost without increasing an environmental load in the manufacturing process.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

[0001] The present invention relates to a casting having a hollow portion formed by a wall portion of a sand mold.

[0002] When a molded product having a space on the inside (that is, having a hollow portion), such as a trough-like product or a pipe, is manufactured by a simple casting process, as molds, a core for forming the hollow portion is used in addition to a master mold.

[0003] For example, in order to obtain a hollow casting 30 shown in FIG. 3, a sand mold 20 shown in FIG. 2 (perspective view) and FIG. 9 (sectional view) can be used. The mold 20 consists of an upper mold 21, a lower mold 22, and a core 23, and is formed with a cavity 29 corresponding to the hollow casting 30. Generally, after molten metal is poured into the cavity 29 and is solidified, the mold 20 and the core 23 are collapsed, whereby the hollow casting 30 can be obtained. As the core 23, for example, a molded object formed into a desirable shape by hardening, for example, sand with a thermosetting resin being used as a binder is used.

[0004] In the case where the shape of hollow casting is more complex, there arises a problem in that the upgrading of quality of casting surface desired for the product of hollow casting is limited, so that the market requirements cannot be met. In addition, there arises an unsolved problem in that the manufacture and removal of core take much time and labor, and the manufacturing cost is high.

[0005] Hereunder, problems with the conventional manufacturing method for a hollow casting will be described by taking an intake manifold, which is one of automotive air intake system parts, as an example of a hollow casting having a more complex shape.

[0006] The intake manifold is an intake pipe which is formed by using, for example, an aluminum alloy as a principal raw material and is used to supply air to cylinders of an engine. FIG. 4 is a perspective view showing an example of the intake manifold. An intake manifold 40 is an intake pipe for a four-cylinder engine, which is formed by four branch pipes 49 which are branched from a surge tank 48 and is connected to each intake port of cylinder of the engine. In order to increase the intake efficiency of air-fuel mixture viewed from the engine, the surface of a wall portion facing a hollow portion 46 is required to be smoothed from the viewpoint of the role of the intake manifold 40. To meet this requirement, in casting the intake manifold, a core having a smooth surface has conventionally been used.

[0007] However, in order to make the surface of core smooth, the core must be formed by using sand etc. having small grain size, and must be subjected to surface treatment such as coating with a facing material, so that the preparation of core requires a high cost. This means an increase in manufacturing cost of the intake manifold. On the other hand, for the core whose surface is coated with the facing material, gas generated from the resin for hardening the sand etc. is difficult to be released, so that the core must be made in a hollow form in which the thickness is decreased to the utmost. This decreases the core strength, and hence a crack is liable to be generated on the surface during handling. For the intake manifold manufactured by using the core with a crack generated on the surface, molten metal intrudes into a cracked portion, and burrs are produced on the surface of the wall portion facing the hollow portion 46, so that the surface becomes rather rough. Therefore, since the smoothness of the core surface has a certain limit, the smoothness of the surface of the wall portion facing the hollow portion of intake manifold also has a limit.

[0008] In the case of a hollow casting having a shape like the intake manifold 40, since the hollow portion is bent, joined, and separated, when the core is removed, a part of core is liable to remain in the hollow portion. Therefore, the core is once collapsed and taken out by giving shocks to the hollow casting or by burning sand, and after appearance finishing etc. are performed, the remaining core sand must again be removed. At this time, the remaining core sand is difficult to be removed completely by simple shot blasting operation. Therefore, conventionally, the remaining sand has been removed from the hollow portion by horning, in which silica sand and beads are mixed and ejected at a predetermined pressure. In particular, for the intake manifold 40, there is a problem in that even if horning etc. are performed from one opening of the hollow portion 46, it is difficult to remove the sand remaining on the inner peripheral side of a bent portion of the hollow portion.

[0009] In this case as well, the initial equipment cost and operation cost of a compressor and a dust collector for performing horning etc. are needed, which leads to an increase in manufacturing cost of the intake manifold. Also, the removed core sand or fine particles produced by a horning material etc. require disposal cost. Further, when cutting is performed in the subsequent process for manufacturing the intake manifold, residual matters, if any, contaminate a cutting oil, and damage a cutter blade, so that the change interval becomes short, which not only increases wastes but also increases the manufacturing cost.

[0010] The above is a description of problems with the related art which has been given by taking the intake manifold as an example of a hollow casting. The present invention has been achieved in view of the above situation, and accordingly an object thereof is to provide a lower-cost and high-quality hollow casting having a hollow portion formed by a wall portion, in which the surface of casting wall portion forming the hollow portion is smooth, and an environmental load is not increased during the manufacturing process.

[0011] Studies on the hollow casting have been carried on, and in particular, the conventional manufacturing method including a process for removing a core after casting has been reviewed and researches thereon has been carried out. As a result, it has been found that the above-described object can be achieved by means described below.

SUMMARY OF THE INVENTION

[0012] That is, according to the present invention, there is provided a hollow casting having a hollow portion formed by a wall portion, in which a surface roughness Ra of a wall portion surface forming the hollow portion is 10 μm or less.

[0013] Also, as the hollow casting, there is provided any one part selected from a vehicular air intake system parts group consisting of an intake manifold, turbine housing, compressor cover, cylinder head, and air duct.

[0014] Also, according to the present invention, there is provided a manufacturing method for a hollow casting, in which a casting having a hollow portion formed by a wall portion is manufactured, wherein after molten metal is poured into a mold to be molded by casting, processing of removing residual matters on a casting wall portion surface forming the hollow portion and processing of smoothing the casting wall portion surface forming the hollow portion are performed at the same time. This is realized by providing a process in which after the molding by casting, a smoothing material is put in the hollow portion and the casting is oscillated.

[0015] Further, according to the present invention, there is provided a method of smoothing the internal surface of a hollow casting, in which a casting wall portion surface forming a hollow portion of a casting having the hollow portion formed by a wall portion is smoothed, wherein a smoothing material is put in the hollow portion and the casting is oscillated.

[0016] In the method of smoothing the internal surface of a hollow casting in accordance with the present invention, a frequency of the oscillation is preferably about 0.5 to 20 Hz, a stroke of the oscillation is preferably about 30 to 200 mm, and the total number of oscillation time of the oscillation is preferably about 3 to 120 minutes.

[0017] Further, a percentage by volume of the smoothing material put in the hollow portion to the hollow portion formed by a wall portion is preferably about 5 to 50%, and the smoothing material is preferably one or two or more mixtures containing at least metal balls or cut wires.

[0018] In the method of smoothing the internal surface of a hollow casting in accordance with the present invention, as a principal material constituting the casting, cast iron or an aluminum alloy for casting can be used.

[0019] Further, there is provided an apparatus for smoothing the internal surface of a hollow casting, in which a casting wall portion surface forming a hollow portion of a casting having the hollow portion formed by a wall portion is smoothed, wherein the apparatus comprises a bed capable of fixing a casting in which a smoothing material is put in the hollow portion in an arbitrary direction and oscillating means for oscillating the bed. At this time, as the oscillating means, means having a prime mover and a crank connected to the prime mover can be used.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a block flow diagram of steps, showing one example of a manufacturing method for a hollow casting in accordance with the present invention.

[0021]FIG. 2 is a perspective view showing one example of a mold for a hollow casting, in which a half is cut and seen through.

[0022]FIG. 3 is a perspective view showing one example of a hollow casting seen through.

[0023]FIG. 4 is a perspective view showing one embodiment of an intake manifold, which is one example of a hollow casting.

[0024]FIG. 5 is a perspective view showing one embodiment of a cylinder head and an intake manifold which is one example of a hollow casting.

[0025]FIG. 6 is a sectional view showing one embodiment of a method of smoothing the internal surface of a hollow casting in accordance with the present invention.

[0026]FIG. 7 is a perspective view showing one embodiment of an apparatus for smoothing the internal surface of a hollow casting in accordance with the present invention.

[0027]FIG. 8 is a view in which a hollow casting used in a working example is cut by a horizontal plane, and a hollow portion is exposed.

[0028]FIG. 9 is a sectional view showing one example of a mold for a hollow casting.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0029] Embodiments of a hollow casting and a manufacturing method therefor, and a method and apparatus for smoothing the internal surface of a hollow casting in accordance with the present invention will now be described in detail successively. The present invention is not construed as being limited to these embodiments, and various changes, modifications and improvements can be made based on the knowledge of those skilled in the art without departing from the scope of the present invention.

[0030] First, a hollow casting in accordance with the present invention will be described.

[0031] A hollow casting in accordance with the present invention is a casting molded by a casting process, and a molded product having a hollow portion formed by a wall portion. The hollow casting in accordance with the present invention is characterized in that the surface roughness Ra of the wall portion surface facing the hollow portion is 10 μm or less. In this description, the wall portion surface facing the hollow portion is described as the inside surface of hollow casting or simply as the inside surface. The surface roughness means surface roughness defined in JIS B0601 “Surface Roughness—Definition and Marking”, and the surface roughness Ra means arithmetic average roughness defined in JIS B0601.

[0032] The hollow casting whose internal surface has very high smoothness of surface roughness Ra of 10 μm or less can be used suitably as a trough-like product or a duct because the pressure loss of a fluid flowing in the hollow portion of this hollow casting is low. Also, because of being a casting, this hollow casting can be molded into a desirable shape according to a mold.

[0033] As suitable applications of the hollow casting in accordance with the present invention, which has the above-described features, vehicular air intake system parts can be cited. The vehicular air intake system parts means parts for a system for supplying air and air-fuel mixture to an engine cylinder, including an intake manifold, turbine housing, compressor cover, cylinder head (intake and exhaust port), and air duct. By these parts having the feature of hollow casting in accordance with the present invention, the intake resistance of a portion reaching the engine cylinder can be decreased, so that even on the same engine, the output thereof can be increased.

[0034]FIG. 5 is a sectional view mainly showing a cylinder head of an engine. A cylinder head 52 having an intake port 54 and an exhaust port 55 is provided on a cylinder 51 in which a piston moves vertically, and further an intake manifold 53 is connected to the cylinder head 52. For example, air filtered by an air cleaner, not shown, passes through the intake manifold 53, and is mixed with fuel by a fuel injector, not shown, or the like to form an air-fuel mixture. The air-fuel mixture is supplied into the cylinder 51 through the intake port 54 in the cylinder head 52 by the opening operation of an intake valve, and is burned by an ignition plug, not shown.

[0035] The air duct is an intake pipe for connecting the air cleaner to the intake manifold. Also, the turbine housing is an impeller chamber for converting exhaust gas into rotational power on an automobile provided with a turbocharger. The compressor cover is an impeller chamber for sending intake gas under pressure.

[0036] The hollow casting in accordance with the present invention is preferably cast by using cast iron or an aluminum alloy for casting as a principal raw material. This is because the hollow casting made of such a raw material has high castability such as high fluidity and high solidification shrinkage property, and thus is less liable to produce casting defects. Also, this is because it has high mechanical properties such as tensile strength, elongation, and toughness.

[0037] The cast iron means an iron-carbon alloy containing a predetermined amount of carbon. The type of cast iron is not subject to any restriction, but it is preferable that nodular graphite cast iron having higher mechanical properties be used. For the aluminum alloy for casting, various types are available according to whether heat-treated or not, contained other elements, and their composition. The type of aluminum alloy is not subject to any restriction. However, it is preferable that an aluminum alloy specified as JIS symbol AC in Japanese Industrial Standard be used. For example, AC4C and AC3A or the like can be cited.

[0038] Next, the method of smoothing the internal surface of a hollow casting in accordance with the present invention will be described.

[0039] The method of smoothing the internal surface of a hollow casting in accordance with the present invention is a method of smoothing the casting wall portion surface forming the hollow portion of the casting having the hollow portion formed by the wall portion. The method of smoothing the internal surface of a hollow casting in accordance with the present invention is characterized in that smoothing materials are put in the hollow portion formed by the wall portion and the casting is oscillated.

[0040] The smoothing materials having a hardness suitable for the material forming the hollow casting are put in the hollow portion formed by the wall portion, and the opening of the hollow portion is closed. The hollow casting is oscillated under preferable oscillating conditions of oscillation amplitude, frequency, and oscillation time as described below by using an apparatus for smoothing the internal surface of a hollow casting, described later, by which the smoothing materials repeatedly collide with the internal surface of the hollow casting. Thereby, the smoothing materials slightly crush and deform the internal surface of hollow casting by means of grinding and striking due to oscillation to improve the smoothness of the internal surface of hollow casting. In this description, the frequency means the number of oscillations repeated per unit time, and the unit thereof is hertz (Hz).

[0041] In the case where means for using a core having a smooth surface in the conventional method is used, the surface roughness Ra of the internal surface of hollow casting is about 8.8 μm at the lower limit, and about 12.5 to 15 μm on an average. According to the method of smoothing the internal surface of a hollow casting in accordance with the present invention, the surface roughness Ra of the internal surface of hollow casting can easily be made 10 μm or less.

[0042] Also, there is no need for prepare a core by being molded using sand having a specially small grain size and by applying a facing material as in the case of the conventional method to enhance the smoothness of the internal surface, so that the cost for manufacturing the core can be reduced, and also the possibility of burrs being formed by the development of a crack on the core can be reduced significantly.

[0043] When the hollow casting is oscillated, the opening of the hollow portion is closed. In this case, it is preferable that an external space following the hollow portion be formed in the end portion. If the opening of the hollow portion is closed without the formation of the external space, the smoothing materials are difficult to move to the opening end portion, so that the surface roughness of the opening end portion is undesirably not improved. An example of the external space is shown in FIG. 6. FIG. 6 is a sectional view showing a state in which the opening of a branch pipe 49 of an intake manifold 40 shown in FIG. 4 is closed by a cover 61. Since an external space 62 is formed by the cover 61, the smoothing materials put in the hollow portion 46 are moved freely even in the opening end portion of the hollow portion 46 by oscillating the intake manifold 40, so that the surface of a wall portion 45 can be smoothed uniformly. All of the opening end portions are closed.

[0044] The smoothing materials preferably contain at least slightly larger (smaller than the diameter of hollow portion) metal balls or cut wires smaller than the metal balls. The metal balls or cut wires may be used without being mixed with other materials, or may be used as a mixture of two or more materials by being mixed with metal particles, abrasives, dry sand, etc. More preferably, a mixture containing at least slightly larger metal balls is used. Also, metal balls having a different size are preferably used. It can be thought that by mixing smoothing materials having a different size, the smoothing materials repeats movement with respect to the internal surface of hollow casting uniformly and thoroughly, and also the internal surface of hollow casting is pressed by the slightly larger metal balls, by which the smoothness of the internal surface of hollow casting can be improved.

[0045] The diameter of metal ball or the length of cut wire, or the material forming metal ball or cut wire may be determined by considering the material forming the hollow casting, the cross-sectional area of hollow portion of the hollow casting, etc., and is not subject to any restriction. For example, in the case of a vehicular air intake system part in which the hollow casting is formed of an aluminum alloy, steel balls or stainless steel balls having a diameter of 10 to 20 mm and stainless steel cut wires having a diameter of 0.6 to 1.2 mm and a length of 0.6 to 1.2 mm can be used suitably.

[0046] Also, the above-described smoothing materials are preferably put in the hollow portion so that the volume thereof is about 5 to 50% of the volume of hollow portion of the hollow casting. The purpose of this is to ensure that the smoothing materials move freely within the hollow portion, and the number of collisions of the smoothing materials with the internal surface of hollow casting is maintained. If the volume of smoothing materials is less than 5%, although the smoothing materials move freely within the hollow portion, the volume of smoothing materials is too low with respect to the area of the internal surface (wall portion surface) of hollow casting. As a result, the number of collisions of the smoothing materials with the internal surface of hollow casting and the pressurizing force are not maintained, so that the internal surface of hollow casting is undesirably difficult to become smooth. If the volume of smoothing materials is more than 50%, the range in which the smoothing materials move freely within the hollow portion is limited, and thus the number of collisions of the smoothing materials with the internal surface of hollow casting and the pressurizing force are not maintained, so that similarly, the whole of the internal surface of hollow casting is undesirably difficult to become smooth.

[0047] Hereunder, the oscillation of the hollow casting for colliding the smoothing materials with the internal surface of hollow casting will be described. In the present invention, the oscillating conditions are not subject to any restriction, but the following conditions are more desirable.

[0048] The frequency is preferably about 0.5 to 20 Hz. The purpose of this is to maintain the number of collisions of the smoothing materials with the internal surface of hollow casting per unit time. If the frequency is lower than 0.5 Hz, the number of collisions of the smoothing materials with the internal surface of hollow casting is not maintained, so that the smoothing materials cannot smooth the internal surface of hollow casting thoroughly, and thus the surface roughness cannot undesirably be improved. Also, depending on the number of smoothing materials (for example, steel balls), even if the frequency is higher than 20 Hz, the effect of improving surface roughness is small, and the effectiveness of energy consumed to increase the frequency undesirably decreases.

[0049] Also, the amplitude of oscillation is preferably about 30 to 200 mm. The purpose of this is to maintain the number of collisions of the smoothing materials with the internal surface of hollow casting per unit time by properly setting the movement range of smoothing materials within the hollow portion. If the amplitude of oscillation is smaller than 30 mm, the number of collisions of the smoothing materials with the internal surface of hollow casting is not maintained, so that the smoothing materials cannot smooth the internal surface of hollow casting thoroughly, and thus the surface roughness cannot undesirably be improved. Even if the amplitude of oscillation is larger than 200 mm, the time during which the smoothing materials are in contact with the internal surface of hollow casting is merely lengthened, and the number of collisions of the smoothing materials with the internal surface of hollow casting does not increase, so that the effect of improving the surface roughness is not great.

[0050] Furthermore, the total number of oscillation time is preferably about 3 to 120 minutes. The purpose of this is to maintain the total number of collisions of the smoothing materials with the internal surface of hollow casting. If the total number of oscillation time is shorter than 3 minutes, the total number of collisions of the smoothing materials with the internal surface of hollow casting is not maintained, and thus the smoothing materials cannot smooth the whole of the internal surface of hollow casting, which undesirably causes variations in surface roughness. Also, even if the total number of oscillation time is longer than 120 minutes, the effect of improving the surface roughness is small, and the effectiveness of time taken to manufacture the hollow casting undesirably does not increase.

[0051] In the method of smoothing the internal surface of a hollow casting in accordance with the present invention, it is preferable that the direction in which the hollow casting is oscillated be selected so that the number of collisions of the smoothing materials with the internal surface of hollow casting is maintained. The oscillating direction differs according to the shape of hollow portion of the hollow casting. For example, when the intake manifold 40 shown in FIGS. 4 and 6 is oscillated, it is unpreferable that the intake manifold 40 be oscillated in the lengthwise direction of the hollow portion 46 thereof, that is, in the direction indicated by the arrow mark Q or P in FIG. 6. This is because the percentage of smoothing materials having a long movement distance in the hollow portion 46 increases, and the number of collisions of the smoothing materials with the internal surface of hollow casting caused by the oscillation decreases. The preferable oscillating direction when the intake manifold 40 is oscillated is the direction indicated by the arrow mark S in FIG. 6 or the arrow mark R in FIG. 4. It is also preferable that the direction be changed during oscillation.

[0052] Next, the apparatus for smoothing the internal surface of a hollow casting in accordance with the present invention will be described.

[0053] The apparatus for smoothing the internal surface of a hollow casting in accordance with the present invention is an apparatus for smoothing the casting wall portion surface forming the hollow portion of the casting having the hollow portion formed by the wall portions. The apparatus for smoothing the internal surface of a hollow casting in accordance with the present invention is characterized by having a bed capable of fixing a casting in which the smoothing materials are put in the hollow portion in an arbitrary direction and oscillating means for oscillating the bed.

[0054]FIG. 7 is a perspective view showing one embodiment of an apparatus for smoothing the internal surface of a hollow casting. An apparatus 70 for smoothing the internal surface of hollow casting includes oscillating means 74 and a bed 73 on which a hollow casting is placed. The oscillating means 74 is made up of a prime mover 71 and a crank 72 connected to the prime mover 71. The rotational motion caused by the prime mover 71 is converted into reciprocating motion by the crank 72, by which the bed 73 can be oscillated in the direction indicated by the arrow mark S2. The bed 73 may have a specific shape in the case where the shape of hollow casting and the oscillating direction are fixed, or may be flat. The shape of the bed 73 is not subject to any restriction if the bed 73 can fix the hollow casting in an arbitrary direction.

[0055] For example, in order to oscillate the intake manifold 40 shown in FIGS. 4 and 6 by using the apparatus 70 for smoothing the internal surface of hollow casting, it is necessary only that predetermined smoothing materials be put in the hollow portion 46, the intake manifold 40, in which the opening of hollow portion 46 has been closed while the external space is formed, be fixed on the bed 73 so that the slant direction indicated by the arrow mark S (FIG. 6) coincides with the direction indicated by the arrow mark S2, which is the oscillating direction of the apparatus 70 for smoothing the internal surface of hollow casting, and the prime mover 71 be operated.

[0056] Next, the manufacturing method for a hollow casting in accordance with the present invention will be described.

[0057] The manufacturing method for a hollow casting in accordance with the present invention is a method of manufacturing the casting having the hollow portion formed by the wall portion by a gravity casting process using a master mold of sand mold or metal mold and a core of sand mold. The means for casting process is not subject to any restriction, and includes what is called the die casting process (high-speed injection) and the low-pressure casting process (low-speed injection). The casting process may be any process in which molten metal is poured into a mold having a cavity of a predetermined shape to perform molding. The manufacturing method for a hollow casting in accordance with the present invention is characterized in that after casting is performed by using the master mold and the sand-made core, the core removing processing and the smoothing processing for the wall portion surface facing the hollow portion are performed at the same time.

[0058] As shown in FIG. 1, the manufacturing method for a hollow casting in accordance with the present invention has at least a casting step 1, an appearance finishing step 2, and an internal surface finishing step 3. The outline of the manufacturing method including these steps will be as described below. First, a predetermined material is prepared as a raw material and is melted. Then, the molten metal is subjected to cleaning treatment as necessary. The molten metal is poured into the mold, and is molded by cooling etc. (casting step 1). Next, burrs etc. produced on the obtained molded product (casting) are removed to finish the outside shape (appearance finishing step 2). Subsequently, the core is collapsed and removed to the utmost. Smoothing materials formed of, for example, metal balls and metal particles are put in the hollow portion of the molded product (casting). The molded product (casting) is oscillated to remove the remaining sand and residues and to smooth the internal surface (internal surface finishing step 3). Thereafter, heat treatment is sometimes carried out as necessary to improve the mechanical properties of the hollow casting.

[0059] Unlike the conventional method, horning is not performed as a separate process to remove residual matters, and the thorough removal of core and the smoothing of internal surface are carried out at the same time. Therefore, the manufacturing process is shortened, and a compressor and a dust collector used for horning etc. can be simplified and thus the maintenance cost and operation cost for these devices can be reduced. In addition, fine particles produced by horning etc. do not adhere to the internal surface of hollow portion. Therefore, when cutting is performed in the subsequent process, a cutting oil is less liable to be contaminated, and a cutter blade is less liable to be damaged, so that the manufacturing cost of the hollow casting is further reduced, and the production of wastes is restrained.

EXAMPLES

[0060] Next, the present invention will be described in further detail with reference to examples. The present invention is not limited to these examples.

[0061] As hollow castings, four intake manifolds for a three-cylinder engine were prepared by being molded by a casting process with an aluminum alloy being used as a raw material.

Example 1

[0062] Shocks were given to the molded intake manifold by oscillation to collapse and remove the core. Thereafter, forty-one steel balls having a diameter of 10 mm and three steel balls having a diameter of 20 mm were put in the hollow portion, which is a gas supply pipe, as smoothing materials, and the intake manifold was oscillated for 100 minutes, at an oscillation frequency of 8H₂, at a stroke of 60 mm to smooth the hollow portion. The result is given in Table 1. The shape of the intake manifold is shown in FIG. 8 (FIG. 8 is a view in which the intake manifold is cut by a horizontal plane, and a hollow portion is exposed). Also, the oscillating direction was the direction indicated by the arrow mark S3 in FIG. 8.

Comparative Example 1

[0063] The smoothing of the hollow portion was performed by the same method as that in example 1 except that stainless steel shot balls having a diameter of 0.6 mm were used as a smoothing material and the oscillation time was 120 minutes. The result is given in Table 1.

Comparative Example 2

[0064] The smoothing of the hollow portion was performed by the same method as that in example 1 except that silica sand of horning sand was used as a smoothing material and the oscillation time was 120 minutes. The result is given in Table 1.

Comparative Example 3

[0065] The smoothing of the hollow portion was performed by the same method as that in example 1 except that grindstone chips were used as a smoothing material and the oscillation time was 120 minutes. The result is given in Table 1. TABLE 1 Surface Fre- Oscillation Oscillation roughness quency amplitude Smoothing time Ra [Hz] [mm] material [min] [μm] Exam- 8 60 Steel balls 10 mm 100 2.00 ple 1 in dia × 41 Steel balls 20 mm in dia × 3 Com- 8 60 SUS shot balls 120 6.90 para- (0.6 mm in dia) tive exam- ple 1 Com- 8 60 Horning sand 120 9.48 para- (silica sand) tive exam- ple 2 Com- 8 60 Grindstone 120 7.04 para- chips tive exam- ple 3

[0066] As described above, according to the present invention, a hollow casting having a highly smooth internal surface can be provided. As such a hollow casting having a highly smooth internal surface, a vehicular air intake system part such as an intake manifold can be manufactured, and by the provision of this vehicular air intake system part, the output of an engine can be increased. In the manufacturing process to obtain such a hollow casting, a core whose surface is especially smoothed is not used, and the core is not removed by using the conventional means such as horning, but, for example, metal balls are put in the hollow portion and the hollow casting is oscillated to provide smoothness and to remove the core. Therefore, the core can be manufactured at a lower cost, and a problem of the production of burrs etc. ascribed to the core is less liable to arise. Also, the equipment cost and operation cost for horning etc. can be reduced, and the manufacturing process can be shortened. Further, the smoothing materials can be used repeatedly. Since the change interval of cutting oil and cutter blade is lengthened in the subsequent process, the production of wastes can be restrained. In addition, the present invention can contribute to the reduction in environmental load through these effects. 

What is claimed is:
 1. A hollow casting having a hollow portion formed by a wall portion, wherein a surface roughness Ra of a wall portion surface forming said hollow portion is 10 μm or less.
 2. A hollow casting according to claim 1, wherein the hollow casting is any one part selected from a vehicular air intake system parts group consisting of an intake manifold; turbine housing; compressor cover; cylinder head; and air duct.
 3. A manufacturing method for a hollow casting, in which a casting having a hollow portion formed by a wall portion is manufactured, wherein after molten metal is poured into a mold to be molded by casting, processing of removing residual matters on a casting wall portion surface forming said hollow portion and processing of smoothing the casting wall portion surface forming said hollow portion are performed at the same time.
 4. The manufacturing method for a hollow casting according to claim 3, further comprising a process in which after said molding by casting, a smoothing material is put in the hollow portion formed by the wall portion and the casting is oscillated.
 5. A method of smoothing an internal surface of a hollow casting, in which a casting wall portion surface forming a hollow portion of a casting having said hollow portion formed by a wall portion is smoothed, wherein a smoothing material is put in said hollow portion and the casting is oscillated.
 6. The method of smoothing the internal surface of a hollow casting according to claim 5, wherein a frequency of the oscillation is about 0.5 to 20 Hz.
 7. The method of smoothing the internal surface of a hollow casting according to claim 5, wherein a stroke of the oscillation is about 30 to 200 mm.
 8. The method of smoothing the internal surface of a hollow casting according to claim 5, wherein the total number of oscillation time of the oscillation is about 3 to 120 minutes.
 9. The method of smoothing the internal surface of a hollow casting according to claim 5, wherein a percentage by volume of said smoothing material put in said hollow portion to said hollow portion is about 5 to 50%.
 10. The method of smoothing the internal surface of a hollow casting according to claim 5, wherein said smoothing material is one or two or more mixtures containing at least metal balls or cut wires.
 11. The method of smoothing the internal surface of a hollow casting according to claim 5, wherein a principal material constituting said casting is cast iron or an aluminum alloy for casting.
 12. An apparatus for smoothing an internal surface of a hollow casting, in which a casting wall portion surface forming a hollow portion of a casting having said hollow portion formed by a wall portion is smoothed, wherein said apparatus comprises: a bed capable of fixing a casting in which a smoothing material is put in said hollow portion in an arbitrary direction; and oscillating means for oscillating said bed.
 13. The apparatus for smoothing an internal surface of a hollow casting according to claim 12, wherein said oscillating means has a prime mover and a crank connected to said prime mover. 